This disclosure relates to a memory system and a program method thereof. Semiconductor memory devices may be volatile or nonvolatile. A nonvolatile semiconductor memory device may retain data stored therein even at power-off. The nonvolatile memory device may be permanent or reprogrammable, depending upon the fabrication technology used. The nonvolatile memory device may be used for user data, program, and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries.
In one type of memory, a vertical NAND flash memory, a plurality of strings may be connected with a bit line. A selected string and unselected strings may exist at a read or program operation. In the event that a read operation is performed under such a condition that boosting charges of unselected strings are not removed, the probability that read disturbance is generated by hot carrier injection may be high. In general, to remove such boosting charges, a pre-pulse may be applied during a predetermined time before a read operation. For example, one technique of applying a pre-pulse is disclosed in U.S. Pat. Nos. 7,852,676 and 8,107,292, the contents of which are incorporated herein in their entirety. However, the application of pre-pulses tends to slow down the processing speed for a memory, which can be undesirable.